Chapter 13- Sound

Section 13-1: Sound Waves

Sound waves are longitudinal
Consist of compressions (areas of high pressure) & refractions (areas of low pressure)
Sound waves radiate out in spherical wavefronts from a vibrating object.
Pitch is known as the frequency that a sound wave is perceived to have.
Sound waves travel outward in 3 dimensions.
Frequencies between 20 Hz and 20000 Hz can be picked up by humans.
Frequencies lower than 20 Hz are identified as infrasonic.
Frequencies higher than 20000 Hz are identified as ultrasonic.
The speed of the sound wave depends upon the medium.
The speed of sound at sea level & room temperature is about 343 m/s.
The freeuncy shift between the relative motion between an observer and the source of waves is known as hte Doppler Effect.
The Doppler Effect can be seen if a sound source and the detector are moving relative to one another.
Formula for pitch:

$f' = \frac{V + V_d}{V - V_s}$

Intensity

As a sound wave travels thorough a medium,energy is transfered to the next molecule.
Intensity is known as the rate which the energy is transferred.
Intensity is measured in watts per square meter.
Formula for intensity:

$I=\frac{P}{4 \Pi r^2}$

threshold of hearing: the quietest sound audible to humans
- Approx. 1.0 X 10^-12 W/m2
threshold of pain: the loudest sound a typical person can tolerate
- Approx. 1.0 W/m2

Perceived Intensity

The perceived loudness of a sound is dependent of the logarithm of hte intensity.
It is measured in decibels (dB).
The sound level measured in decibels is:
- dB = 10log)_10 intensity
"Intensity" is the intensity of sound being measured.
"Intensityr k"is the relative intensity
Decibel level is a measure of relative intensity in terms of logarithm.
A forced vibration at the natural frequency produces resonance.

Fundamental Frequency- the lowest frequency of vibration of a standing wave.
Fundalemental frequency is the lowest possible frequency of a standing wave.
Harmonics of a vibrating string or pipe open at both ends is found by the equation:

$f_n = n \frac{v}{2L}$
n = 1, 2, 3...

$f_n = n \frac{v}{4L}$
n = 1, 3, 5...

Harmonic Series- a series of frequencies that includes the fundamental frequency and integral multiples of the fundamental frequency.
Timbre- the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities.

The fundamental frequencies produces by a typical piano range from 26.0 Hz to 4435 Hz. What are the wavelengths of these frequencies? (Answer: wavelength= 13.3 m, wavelength= 0.0778 m)
Two cars are moving towards each other, the one on the left at 20.0 m/s and the one on the right at 30.0 m/s. The car on the left sounds its horn with a frequency of 440.0 Hz. What pitch will the driver in the car on the right hear? (pitch= 508 Hz)

Works Cited:
Faughn, Jerry S. and Raymond A. Serway. Physics. New York: Holt, 2004.
Strong, Tom. Course notes. Honors Physics, Dept. of Science, Mount Lebanon High School. May and June 2009.